1. Field of the Invention
The present invention relates to a liquid crystalline material and a liquid crystal display panel. More specifically, the present invention relates to a liquid crystalline material comprising a liquid crystalline molecule which has three fluorine atoms bonded to the terminal end thereof, and to a TN-type liquid crystal display panel using the liquid crystalline material.
2. Description of the Prior Art
Liquid crystal display panels of TN (twisted nematic) type are now being employed in a wide variety of fields. In particular, liquid crystal display panels of so-called active matrix type, in which pixels made from a TN-type liquid crystal are directly operated lay an active element such as a thin film transistor (TFT), have display characteristics comparable to those of CRT. They are therefore extensively applied to liquid crystal televisions, personal computers and the like.
Further, liquid crystal display panels are characterized by low power consumption, so that they are often utilized to portable apparatus such as personal computers of lap-top and notebook types. For such portable apparatus, it is required to make the power consumption of their liquid crystal display panel as low as possible in order to extend the time operable by a battery. To lower voltage required to operate the display panel is one means to fulfill the above requirement, and various studies have been carried out for this purpose.
A liquid crystalline material which is operable with low voltages is now being developed; such a material can be one solution to lower voltage required to operate a liquid crystal display panel.
A liquid crystalline material comprising as a main component a molecule which contains as a polar group one or two fluorine atoms (--F) or --OCF.sub.3 groups at the terminal end thereof has been used as the liquid crystalline material operable with low voltages. However, an on-drive voltage of approximately 5 V is needed to operate a liquid crystal display panel obtained by using such a liquid crystalline material, and it is the present situation that when this liquid crystal display panel is operated by a battery, the battery has to be changed in a short period of time.
Recently, in order to obtain liquid crystalline materials operable with lower voltages, liquid crystalline molecules having an increased anisotropy of dielectric constant .DELTA..epsilon., obtained by increasing the number of polar groups at the terminal end of a molecule which composes a liquid crystal are being put into practical use. Such liquid crystalline molecules include those molecules which have three fluorine atoms bonded to benzene ring positioned at the terminal end thereof. They are represented by the following structural formulas (1), (2) and (3): ##STR2## wherein R represents a linear alkyl group, C.sub.n H.sub.2n+1 in which n is 3, 4, 5, 6 or 7, A represents --CH.sub.2 CH.sub.2 -- or direct bond (--) without --CH.sub.2 CH.sub.2 --, and B represents --CH.sub.2 CH.sub.2 -- or direct bond (--) without --CH.sub.2 CH.sub.2 --.
For instance, liquid crystalline materials I and II shown in FIGS. 1 and 2 have been known as those which contain the above-described molecules.
The liquid crystalline material I shown in FIG. 1 is one which is obtained by incorporating the molecules represented by the structural formulas (1), (2) and (3) into conventional liquid crystalline components. The components (7) to (10) shown in FIG. 1 correspond to the molecules having the formulas (1), (2) and (3). The liquid crystalline material II shown in FIG. 2 is one which contains as main components the molecules represented by the structural formulas (1), (2) and (3). When the liquid crystalline material I or II is practically used as a material for a liquid crystal display panel, a chiral material is added to the liquid crystalline components shown in FIG. 1 or 2. However, the wt % values shown in FIGS. 1 and 2 represent the proportions of the liquid crystalline components in the liquid crystalline materials which contain no chiral material.
These two liquid crystalline materials shown in FIGS. 1 and 2 have a high anisotropy of dielectric constant .DELTA..epsilon. of approximately 7 to 9, and are operable with a lower voltage of approximately 3 V.
However, a liquid crystalline material which is operable with a low voltage of approximately 2.5 V has not been obtained so far even by utilizing any one of the molecules represented by the structural formulas (1), (2) and (3). For instance, even the liquid crystalline material shown in FIG. 2 has an anisotropy of dielectric constant .DELTA..epsilon. of 8.7, and requires an operating voltage of at least 3 V.
For the purpose of decreasing the power consumption of liquid crystal display panels, approximately 3 V is not sufficiently low as voltage to operate a liquid crystalline material, and a liquid crystalline material which is operable with a voltage of lower than 3 V, for example, a voltage of approximately 2.5 V or lower is required.
Next, a pixel in a conventional active element type liquid crystal display panel and a peripheral structure of the display panel are described with reference to FIGS. 11(a) and 11(b). FIG. 11(a) is a plan view showing a pixel of a liquid crystal panel and an arrangement of bus lines, and FIG. 11(b) is a cross sectional view taken generally along a line I--I of FIG. 11(a).
As shown in FIGS. 11(a) and 11(b), a plurality of scanning bus lines 14 formed out of a metal such as Cr are arranged far from each other at prescribed intervals on a first plane of a first transparent substrate 11, and a plurality of data bus lines 19 are arranged far from each other at prescribed intervals on the scanning bus lines 14 through an insulating film (not shown) to cross over the lines 14. The data bus lines 19 extend in a direction perpendicular to another direction in which the scanning bus lines 14 extend.
A pixel electrode 13 is arranged on a portion of the first transparent substrate 11 surrounded by each of the scanning bus lines 14 and each of the data bus lines 19. Each of the pixel electrodes 13 is connected to one of the data bus lines 19 through an active element 20, and each of the active elements 20 is actuated by a voltage applied from one of the scanning bus lines 14. A thin-film transistor (TFT) or a metal-insulator-metal (MIM) diode is used as the active element 20.
The scanning bus lines 14, the data bus lines 19, the active elements 20 and the pixel electrodes 13 are covered by a first alignment film 15A. Also, on a first plane of a second transparent substrate 12, a transparent electrode 16 and a second alignment film 15B are arranged in that order. The first transparent substrate 11 and the second transparent substrate 12 are arranged on condition that the first alignment film 15A face the second alignment film 15B, and a liquid crystal 17 is packed in a region between the first alignment film 15A and the second alignment film 15B. In addition, a polarizing plate 18A is arranged on a second plane of the first transparent substrate 11, and a polarizing plate 18B is arranged on a second plane of the second transparent substrate 12.
To operate the active element type liquid crystal display panel by applying a low voltage to the display panel, the improvement of the display panel has been actively studied. For example, the research of the display panel has been mainly made to improve a liquid crystal material or to lengthen a chiral pitch of a liquid crystal molecule.
However, another type of technical improvement is required as well as the above improvement of the display panel to operate the display panel by applying a much lower voltage to the display panel.